1. Field of the Invention
The current invention is concerned with the controllable generation of multiple optical wavefronts and, for convenience, will be referred to as an active hologram. It has applications in areas such as (for example) three dimensional displays of information, optical interconnects, beam steering, optical pattern recognition, wavefront synthesis and shaping, and diffractive optical elements.
2. Discussion of Prior Art
Computer Generated Holograms (CGH) are a powerful method for generating almost arbitrary optical wavefronts. They represent a generalisation of the conventional hologram (themselves, a powerful technique with potential use in many optical systems). In the latter, however, the wavefront to be stored in the hologram must be realised by some means during the writing of the hologram. In CGH, this restriction does not apply. All that is needed is a mathematical description of the required wavefront. Subsequent calculations determine the structure of the CGH, which can then be fabricated by a variety of techniques.
Typically a CGH is a two dimensional, rectilinear array. Each element (or pixel) of this array has a given optical transmission t.sub.ij, where t.sub.ij can be either real (an absorption modulated CGH) or imaginary (a phase modulated CGH) or a combination of both. Examples of absorption CGH include ones fabricated from photographic film or patterned into chrome on glass. Phase modulated CGH are often made by etching into a transparent substrate, to produce a surface relief pattern.
Using holographic analogies, it is clear that CGH can perform general wavefront transformations. As such, they are a key, enabling technology for many optical systems, particularly optical processing and interconnects.
Switchable diffraction gratings are also known. They rely on the principle of matching the refractive index of some electro optic material (e.g. a nematic liquid crystal) with a transparent substrate. When a voltage is applied across the whole device (through two transparent electrodes on the input and output faces, say) the refractive index of the electro optic material changes accordingly. Consequently there is now an index mismatch, and the light passing through the device will be diffracted at the boundary between the substrate and the electro optic material. The larger the applied voltage, the greater the index mismatch.
Spatial Light Modulators (SLM) are also known (see for example OPTICS LETTERS, Vol. 11. No. 11 November 1986 pp 748-750; ELECTRONICS LETTERS Vol. 28 No. 1, 2 January 1992, pp 26-28). Typically, such a device would comprise an array of pixels each of which has can impart an optical phase modulation of one of two magnitudes (one of which would typically be zero) depending on the voltage applied to that pixel. In order to produce a desired wave front, appropriate voltages are applied to each pixel. Under appropriate computer control, a large number of CGH can be implemented on the SLM. These devices are however, complex, expensive, unreliable and require complex drive circuitry (for example each pixel has to be addressed individually). They require significant computer systems if several CGH are required to be rapidly implemented. Additionally, pixel sizes and counts are limited.